Telephone system



Dec. 12, 1944. G. T.- BAKER TELEPHONE SYSTEM Filed Feb. 6, 1943 INVENTORGEORGE THOMAS BAKER ATTORNEY:

' Patented Dec. 12, 1944 TELEPHONE SYSTEM George Thomas Baker,Liverpool, England, as-

signor, by mesne assigrnnents, to Automatic- Electric, Laboratories,Inc., Chicago, 111., a corporation of Delaware Application February 6,1943, Serial No. 474,905 In Great Britain April 28, 1942 Claims.

The present invention relates to telephone systems and is moreparticularly concerned with telephone systems of the type employing, for

signalling purposes, alternating currents of a frequency within thevoice spectrum.

In such systems account must be taken of th likelihood that currents ofsignalling frequency will be present in the currents representing speechand known means for distinguishing between signalling frequencies andthe same frequencies when occurring as part of a speech passagegenerally depend on the fact that in the latter case if the frequency orfrequencies allocated for signalling are present, they will beaccompanied by other frequencies. The presence of these otherfrequencies serves to render inoperative the receiving device whichresponds to currents of signalling frequencies.

In some circumstances it is found desirable to arrange that signalsshall not pass a particular point in a transmission line while speech,including the signalling frequencies, shall be enabled to pass withouthindrance. Arrangements have already been proposed for meeting thisrequirement and the chief object of the invention is to provide improvedarrangements for permitting speech topass without interference oralteration while barring the passage of signalling frequencies.

According to one feature of the invention, in a telephone systememploying voice frequency currents for the cidental to the setting upand supervision of a connection, blocking means normally effective toobstruct the passage over the line of currents of one or morefrequencies in the voice frequency range are arranged to be controlledin response to currents of non-signalling frequencies to have theirblocking efiect neutralised- According to another feature of theinvention, in a telephone system employing voice frequency currents forthe transmission of signals incidental to the setting up and supervisionof a connection, the line circuit includes a filter device adapted toobstruct the passage of currents of the signalling frequency orfrequencies and to be rendered inoperative in response to the receptionof currents of non-signalling frequencies.

According to a further feature of. the invention, in a telephone systememploying voice frequency currents for the transmission of signalsincidental to the setting up and supervision of a connection, the linecircuit is normally blocked to currents of all frequencies and meansretransmission of signals in-- sponsive to currents ofnon-signallingfrequency are arranged to remove the blocking efiect.

The invention will bebetter understood from the following description offour methods of carrying it into effect, reference being had to theaccompanying drawing comprising Figs. 1-4.

Fig. 1 shows a line circuit for a voice frequency signalling system,using a filter in the line, for blocking the signalling frequencies, andhaving an amplifier controlled relay in a branch of the line forremoving the blocking effect.

Fig. 2 shows schematically an alternate form of the invention using avariable attenuation network in combination with filters.

Fig. 3 showsstill another method of operation by the use of a thermalswitching device, in conjunction with a filter.

Fig. 4 shows a modification of the invention using an echo suppressor incircuit with a filter controlling the operation of the line disablingfeature,

In the arrangements of all the figures it. is

assumed that one frequency only is employed for signalling purposes andit will be understood that in Figs. 1-3 the line is normally blocked asregards this frequency only but'ofiers no appreciable hindrance to otherfrequencies. In

Fig. 4 the line is normally blocked as regards all frequencies and thisblocking is rendered ineffective in response'to the reception ofnonsignalling frequencies either with or without the selected signallingfrequency. 7

Referring now to Fig. 1, the combination of inductances and capacitancesLl, Cl and L2, C2, represents in simple form a filter of the typenecessary to provide high attentuation to a signalling frequency so thatthe line is in effect blocked to such frequency. Connected across theline by way of a pair of load limiting resistances R! and R2'is anamplifier A, in the input circuit of which a similar filter circuitrepresented by L3, L4, C3, C4 is inserted so as to ,bar thesignallingfrequency. The output circuit of the amplifier is connected toa relay K.

The reception of a signalling frequency unaccompanied by otherfrequencies is without effect on the amplifier so that relay K does notoperate and the line effectively remains blocked as far as thisfrequency is concerned. If however it is accompanied by otherfrequencies, then x tinuous speech passage even though at any one timethe speech frequency component may only consist of current of thesignalling frequency,

way basis only, i. e., with the input on the lefthand side and theoutput on the right-hand side, it might be arranged to connect the amplifier A on the outgoing side of the line filter,

in which case it would be unnecessary to employ the additional filter incircuit therewith as shown, since discrimination against a signallingfrequency will be effected by the line filter.

Instead of the mechanical switching arrangement shown, it would bepossible to employ a static switching device comprising forinstance dryplate rectifiers or other units the impedance of which can be varied toproduce the switching effect brought about by the contacts of relay K.This is diagrammatically shown in Fig. 2 where the filters FI and F2 areeach arranged to offer high impedance to the signalling frequency, whilethe static switching device SSD which is connected across the linefilter Fl and which may comprise a variable attenuation network,normally has a high impedance so that the line is normally blocked tothe signalling frequency. If a non-signalling frequency is receivedeither alone or in combination with a signalling frequency, it will passthrough filter FI and also by way of filter F2. and the static switchingdevice SSD. The potential change involved in the latter path is used tobias back the switching device SSD so that it now becomes of lowimpedance and serves to pass all frequencies.

In the arrangement of Fig. 3 there is connected in the line a small beadof known material such as uranium oxide or silver sulphide which has anon-linear negative temperature coefficient, that is to say itsresistance decreases rapidly from its normal high value with increase oftemperature: this will be referred to as a thermal switching device andis designated TSD. The filter F3 which is arranged to offer highimpedance to the signalling frequency is connected in series with theheater winding for this device, and hence if a signalling frequencyalone is'received it will be unable to pass through since one paththerefor'is interrupted by the high resistance of the bead TSD, whilethe other path is interrupted by virtue of the action of the filter F3.If, on the other hand, a non-signalling frequency alone or accompaniedbythe signalling frequency is received, it will be able to find a path byway of the filter F3. Hence the current quencies.

Referring now to Fig. 4, use is made in this instance of an echosuppresser ES which is connected to the line in circuit with the filterF4 which serves to bar the signal frequency. If a signal frequency onlyis received, the equipment ES will not come into operation and the linewill remain blocked, while if a non-signalling frequency is received theecho suppressor comes into An advantage of the above-describedarrangements is that since the line is normally blocked to signalfrequencies it is impossible for unwanted pips of signal tone to getthrough to another circuit where trouble might be caused.

tion with Figs.' 1-3 it will be seen that the line is never blocked tofrequencies other than those selected for signalling purposes and henceeven if the signal barring device'should fail to come into operation, aspeech passage could be transmitted through without more interferencethan would be caused by the suppression of frequencies which correspondto those used for signalling purposes.

What I claim as new and desire to secure by Letters Patent is:

1. In a telephone system employing voice frequency currents for thetransmission of signals, a line for the setting up and supervision of asignalling and talking connection, means normally efiective to obstructthe passage over said line of currents of one or more frequencies in thevoice frequency range, and means responsive to currents ofnon-signalling frequencies to remove said obstruction to allowtransmission of said frequencies over the line.

2. In a telephone system in which there is means for transmittingcurrents of voice 'frequency over a line to transmit signals incidentalto the setting up and supervising a signalling and talking connectionover the line, a filter tuned to normally obstruct the passage over saidline of said currents of the signalling frequency or frequencies, andmeans for removing said filter obstruction responsive to thetransmission of currents of non-signalling frequencies over said line.

. 3. In a telephone system, a line circuit over which voice frequencysignalling currents are transmitted during the setting up of a connection over the line, an amplifier associated with said line, a filter inthe inputof said amplifier tuned to make the amplifier unresponsive tosaid signalling currents but responsive to currents of otherfrequencies, and means in the output of said amplifier for controllingsaid line circuit when said other frequencies are received.

operation.and either statically or by a relay T operation as indicatedby contacts so, extends the line through for all frequencies.

4. A system as claimed in claim 3, in which said last means comprises arelay connected to the output terminals of said amplifier, and adjustedto operate on the transmission of currents of frequencies receptive tosaid amplifier over said line.

5. In a telephone system employing voice frequency currents for thetransmission of signals, a line for the setting up and supervision of asignalling and talking connection, means including a line filter and ashunt normallyiblocking said line to' currents of a signalling frequencyor frequencies, an amplifier, a slow to release relay, said relayoperating during transmission of non-signalling currents through saidamplifier to disable and cause said blocking means to remain disabledduring transmission of speech currents over said line, even though thespeech frequenc component consists of currents of the signallingfrequency or frequencies.

6. In a signalling system, a line circuit over which speech currents areto be transmitted, andover which certain control frequencies are alsotransmitted, a filter in the line circuit tuned to block the controlfrequencies from passing over the line, a device associated with theline and responsive to frequencies of current other than the controlfrequencies to render the filter in the line circuit ineffective.

7. A system such as claimed in claim 6, in which said device includes avariable attenuation network in shunt of the line circuit filter, andmeans for varying the attenuation of the network to vary its shuntefiect.

8. A system such as claimed in claim 6, in which said filter comprisesan element of resistance material having a high negative coefiicient ofresistance change with temperature, and means consisting of a heatingcoil in series with a second filter to render said first filterineffective.

9. In a telephone system, a line circuit over I which voice frequencysignalling currents are transmitted incidental to the setting up of aconnection over the line, means normally operafive to obstruct thepassage over said line of all frequency currents, and means responsiveto the transmission of non-signalling frequencies over the line forremoving the obstruction.

10. In a telephone system, a line circuit'over which voicefrequency'signalling currents are transmitted during the setting up of aconnection over the line, means blocking said line to all frequenccurrents, a control branch of said line circuit, a filter in saidcontrol branch, a device operating over said control branch in responseto the transmission over the line circuit of currents of non-signallingfrequencies for removing the blocking means to permit the transmissionof all frequency currents over the line.

GEORGE THOMAS BAKER.

